Switching Diodes Basics: Working, Types and Circuit Analysis

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Ⅰ Introduction

Switching Diodes are a type of semiconductor diode. They are specially designed and manufactured for "on" and "off" on a circuit. As the name suggests, it refers to a diode with a switching function. This diode has the performance of passing current (ON) when voltage is applied in the forward direction and stopping (OFF) current when the voltage is applied in the reverse direction. Compared with other diodes, the reverse recovery time (trr) is short, that is, the time that the switching diode takes from the on-state to the fully off-state is short. Common Switching Diodes have 2AK, 2DK, and other series, mainly used in electronic computers, pulses, and switching circuits.

Ⅱ Working principle of Switching Diodes

When the semiconductor diode is turned on, it is equivalent to the switch being closed (the circuit is turned on). When it is turned off, it is equivalent to the switch being opened (the circuit is turned off). Due to the unidirectional conduction characteristics of semiconductor diodes, the PN junction is turned on under positive bias, and the resistance in the on-state is very small, about tens to hundreds of ohms; under reverse bias, it is turned off, and its resistance is very large. The general silicon diode is above 10MΩ, and the germanium tube also has tens of thousands of ohms to hundreds of thousands of ohms. Using this feature, the diode will play a role in controlling the current on or off in the circuit, making it an ideal electronic switch.

Switching diode structure

The above description applies to any ordinary diode or the principle of the diode itself. But for switching diodes, the most important feature is the performance at high frequencies. Under high-frequency conditions, the barrier capacitance of the diode exhibits extremely low impedance and is connected in parallel with the diode. When the capacitance of this barrier capacitor reaches a certain level, it will seriously affect the switching performance of the diode. Under extreme conditions, the diode will be short-circuited. High-frequency current no longer passes through the diode, but directly bypasses the barrier capacitance, and the diode fails. The barrier capacitance of the switching diode is generally very small, which is equivalent to blocking the path of the barrier capacitance, which achieves the effect of maintaining good unidirectional conductivity under high frequency.

Schematic diagram of switching diodes

Ⅲ Working characteristics of Switching Diodes

The time from switching off (high resistance state) to conducting (low resistance state) of the switching diode is called the turn-on time. The time from turning on to the end is called the reverse recovery time. The sum of the two times is called the switching time. Generally, the reverse recovery time is greater than the turn-on time, so only the reverse recovery time is given in the operating parameters of the switching diode. The switching speed of the switching diode is quite fast. The reverse recovery time of the silicon switching diode is only a few nanoseconds. Even the germanium switching diode, its reverse recovery time is only a few hundred nanoseconds.

The switching diode has the characteristics of fast switching speed, small size, long life, and high reliability. It is widely used in switching circuits, detection circuits, high-frequency and pulse rectification circuits, and automatic control circuits of electronic equipment.

When the forward voltage is applied to the two poles of the switch, the diode is in the on-state, which is equivalent to the on-state of the switch. When a reverse voltage is applied to the switching diode, the diode is in the off state, which is equivalent to the off state of the switch. Switching Diodes use this feature to achieve better switching characteristics, faster-switching speed, the smaller junction capacitance of the PN junction, smaller internal resistance during conduction, and greater resistance when off.

(1) Turn-on time. It takes time for the switching diode to turn on from the cut-off, which is called the turn-on time. The shorter this time, the better.

(2) Reverse recovery time. After the switching diode is turned on, the forward voltage is removed. The time required for the diode to turn from on to off is called the reverse recovery time. The shorter this time, the better.

(3) Switching time. The sum of turn-on time and reverse recovery time is called switching time. The shorter this time, the better.

Ⅳ Types of Switching Diodes

Switching diodes are divided into ordinary switching diodes, high-speed switching diodes, ultra-high-speed switching diodes, low-power switching diodes, high back-pressure switching diodes, silicon voltage switching diodes, and so on. The package form of the switching diode includes a plastic package and a surface package. 

Switching diode shape

1 Ordinary switching diode

Commonly used general Switching Diodes are 2AK series germanium switching diodes. The table below is the main parameters of the 2AK series Switching Diodes.

The main parameters of 2AK series switching diodes

2 High-speed switching diode

High-speed Switching Diodes have a shorter reverse recovery time than general switching diodes and have faster on and off frequencies. Commonly used domestic high-speed switching diodes are the 2CK series, 1N series, 1S series, 1SS series (leaded plastic package), and RLS series (surface mount).

High-speed diode model parameters

3 Ultra-high-speed switching diode

Commonly used ultra-high-speed diodes are 1SS series (leaded plastic package) and RLS series (surface package).

Ultra-high-speed switching diode model parameters

4 Low power switching diode

Low-power switching diodes have lower power consumption, but their zero-bias capacitance and reverse recovery time values are lower than those of high-speed Switching Diodes. Commonly used low-power Switching Diodes are RLS series (surface package) and 1SS series (leaded plastic package).

Low power switching diode parameters

5 High backpressure switching diode

The reverse breakdown voltage of the high reverse voltage switching diodes is above 220V, but it's zero bias capacitance and the reverse recovery time value are relatively large. Commonly used high backpressure switching diodes are RLS series (surface package) and 1SS series (leaded plastic package).

Model parameters of the high back pressure switching diode

6 Silicon voltage Switching Diodes

Silicon voltage switching diodes are a new type of semiconductor device, which is divided into unidirectional voltage Switching Diodes and bidirectional voltage switching diodes. They are mainly used in flip-flops, overvoltage protection circuits, pulse generators and high-voltage output, delay, electronic switches, and other circuits.

Main parameters of two commonly used silicon voltage switching diodes

Unidirectional voltage switching diode outline drawing and circuit graphic symbols

Unidirectional voltage switching diodes are also called turning diodes. They consist of silicon semiconductor materials with a four-layer structure of PnPN. The positive direction is a negative resistance switching (meaning that when the applied voltage rises to the positive turning voltage value, the switching diode changes from the off-state to the on-state, that is, it changes from high resistance to low resistance), and the reverse is the stable characteristic. The bidirectional voltage diode is composed of NPnPN five-layer silicon semiconductor material, and its forward and reverse directions have the same negative resistance switching characteristics.

Outline drawing and circuit graphic symbol of the bidirectional voltage switching diode

Ⅴ Typical application circuit analysis of Switching Diodes

1. The figure below shows a typical diode switching circuit. VD1 in the circuit is a switching diode, and L1 and capacitor C1 form an LC parallel resonant circuit.

(1) When the switch S1 is off, the DC voltage +V cannot be added to the positive pole of VD1. At this time, VD1 is cut off, and the resistance between the positive pole and the negative pole is very large. So C2 cannot be connected to the circuit because of the open circuit of VD1. L1 is paralleling with C1 that constitutes an LC parallel resonant circuit.

(2) When the switch S1 is turned on, the DC voltage +V is applied to the positive electrode of VD1 through S1 and R1 to turn on VD1. The resistance between the positive electrode and the negative electrode is very small, which is equivalent to the connection between the positive electrode and the negative electrode of VD1. In this way, C2 is connected to the circuit and is connected in parallel with the capacitor C1. L1 and C1 and C2 form an LC parallel resonant circuit.

In the above two states, due to the different capacitances in the LC parallel resonant circuit, one case is the only C1, and the other case is C1 and C2 in parallel. When the capacitance is different, the resonance frequency of the LC parallel resonant circuit is different. Therefore, the real role of the circuit where VD1 is located is to control the resonance frequency of the LC parallel resonance circuit.

When there is a switch in the circuit, the analysis of the circuit takes the case of the switch on and off as an example to analyze the working state of the circuit. Therefore, when switching elements appear in the circuit, they can provide ideas for circuit analysis. The signal in the LC parallel resonant circuit is added to the positive pole of VD1 through C2. But because the amplitude of the signal in the resonant circuit is relatively small, the positive half-cycle signal amplitude applied to the positive pole of VD1 is very small and will not make VD1 conductive.

2. Analysis of the working principle of similar circuits

As shown in the figure, VD1 in the circuit is a switching diode, and the control voltage is applied to the positive electrode of VD1 through R1. The control voltage is a rectangular pulse voltage, and the waveform is shown in the figure.

When the control voltage is 0V, VD1 cannot be turned on. That is equivalent to an open circuit. At this time, it does not affect the L1 and C1, and L2 and C2 circuits. When the control voltage is high, the control voltage turns on the switching diode VD1. The AC signal at point A in the circuit is grounded through the conducting VD1 and capacitor C3, which is equivalent to AC grounding at point A in the circuit, making the L2 and C2 circuits inoperative.

From the above analysis, it can be seen that the diode VD1 in the circuit is equivalent to a switch that controls whether the AC signal at point A in the circuit is grounded or not.

Ⅵ How to test Switching Diodes?

1. To test the polarity

Switching the multimeter in the R×100 or R×1k range. The two test leads should be connected to the two electrodes of the diode respectively. After the first test, reverse the two test leads and test again. Among the results of the two tests, one a larger resistance value (reverse resistance) and another a smaller resistance value (forward resistance). In a test with small resistance, the black test lead is connected to the anode of the diode, and the red test lead is connected to the cathode of the diode.

2. The detection of single negative conductive performance and the judgment of good or bad

Generally, the forward resistance value of the germanium material diode is about 1kΩ, and the reverse resistance value is about 300. The resistance value of the silicon material diode is about 5 kΩ, and the reverse resistance value is  ∞ (infinity). The smaller the forward resistance, the better, and the larger the reverse resistance, the better. The greater the difference between the forward and reverse resistance values, the better the unidirectional conductivity of the diode. If the measured forward and reverse resistance values of the diode are close to 0 or the resistance value is small, it means that the diode has broken down a short circuit or is damaged. If the measured positive and negative resistance values of the diode are both infinite, it means that the diode has been opened and damaged.

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